hmm.. thanx for the answers.
As I thought it seems counterproductive to use aliasing..
Lets rephrase the second question :

the idea is to have the result of the parsing as internal object data but not global..
this way I can create many objects that can parse different string-flows.. so that i do :

my $obj = new Parser();
$obj->parse;
print "$_\n" for @{$$obj{_data}};
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this mean I have to write directly into $$obj{_data} array during the parse..
I can't figure out how to access object data from the parse..probably class var which will work as intermediate reference...

package Parser;
our $ref;

my $grammar = q{
    { my $data = $ref }
    start : ..... 
    ruleX : blah hmm { push @{$$data{_data}}, $item[2] }
}

sub parse {
   my $self = shift;
   $ref = $self{_data};
   $parser = new Parse::RecDescent;
   $parser->start($$self{str})
   ...
}
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havent tested..is this the correct way of doing it ?! what bothers me in this variant is that $ref can be overwriten from different objects..(which can be problem in multiuser env., not that i will use it in such way but who know :"))
Using array/stack will not solve this problem either..